1. Field of the Invention
The present invention relates to a pollutant removal technique for removing pollutants adhered to a surface of an optical member. In particular, the present invention is preferably usable to remove pollutants adhered to a surface of an optical member included in an illumination optical system or a projection optical system of a projection exposure apparatus which uses, for example, an excimer laser light source as an exposure light source.
2. Description of the Related Art
In relation to members classified into optical members which are not limited to optical members for the projection exposure apparatus, the presence of foreign matters adhered to the surface causes, for example, the uneven illuminance and the decrease in the transmittance of the light beam, and the optical performance, which is to be originally possessed by the optical element, is not obtained. Therefore, various countermeasures have been made thereagainst.
The following methods have been adopted to avoid the water droplet adhesion on the members such as spectacle lenses and front glasses of automobiles which dislike the cloudiness principally caused by water droplets. That is, a cloudiness-preventive agent containing a surfactant is applied to the uppermost layer (see, for example, Japanese Patent Application Laid-open No. 56-90876). The surface roughness is made coarse to increase the surface area so that the wettability is increased thereby to avoid the cloudiness (see, for example, Japanese Patent Application Laid-open No. 55-154348). The coating is formed with a substance including fluorine or with a hydrophobic polymer (see, for example, Japanese Laid-Open Patent Publication No. 54-74291).
On the other hand, the pulse laser beam such as the excimer laser is used in some situations as the exposure light beam in the projection exposure apparatus which is employed to transfer a pattern of a reticle as a mask onto respective shot areas of a wafer (or a glass plate or the like) applied with a resist as a photosensitive substrate (sensitive object) during the lithography step to produce devices including, for example, semiconductor elements and liquid crystal display elements. However, for example, if any organic substance is present in a minute amount in the gas on the optical path for the pulse laser beam, a thin film-shaped pollutant is adhered to the surface of the optical member on the optical path due to a kind of the photo-CVD (Chemical Vapor Deposition) action. As a result, it is feared that the transmittance may be gradually decreased, and/or the uneven illuminance may appear.
In view of the above, it has been investigated that the following method (1) or (2), which has been hitherto used, for example, in the technique for the camera, is adapted to the projection exposure apparatus in order to avoid the adhesion of pollutants onto the surface of the optical member in the illumination optical system and/or the projection optical system.
(1) A titanium oxide film, which serves as a photocatalyst, is formed on the surface of the optical member corresponding to a portion to which the pollutant tends to adhere so that the organic pollutant is decomposed by the photocatalyst action (see, for example, Japanese Patent Application Laid-open No. 8-313705).
(2) A silicon oxide film is densely formed on the optical member to decrease the surface area of the optical member so that it is tried to decrease the amount of the pollutant. Alternatively, an organic film, which has the carbon fluoride group, is formed on the optical member to decrease the tight contact with respect to the pollutant so that the amount of adhesion of the organic pollutant is decreased (see, for example, Japanese Patent Application Laid-open No. 6-5324).
Conventionally, for example, the following countermeasure has been also adopted. That is, an optical member, to which the pollutant has been adhered, is detached from the illumination optical system or the projection optical system to exchange the optical member.
The following problems are pointed out for the conventional techniques as described above.
At first, in relation to the method for forming the titanium oxide film of (1), the titanium oxide film absorbs the short wavelength laser beam such as the KrF and ArF excimer lasers, although the titanium oxide film have a high transmittance with respect to the relatively long wavelength light beam such as the i-ray (365 nm). Therefore, the titanium oxide film exhibits a fairly low transmittance with respect to the laser beam having the short wavelength as described above. For this reason, the exposure time is prolonged to obtain an appropriate exposure amount, and the throughput is lowered in the projection exposure apparatus which uses the exposure light beam of the KrF or ArF excimer laser beam. Further, in the case of the use for a long period of time, the titanium oxide film begins to be destroyed, for example, due to the heat generated by the light absorption. Therefore, it is necessary to exchange the optical member.
On the other hand, the method for densely forming the silicon oxide film on the optical member of (2) is not preferred so much, because the method causes the change of the characteristics of, for example, the antireflection coating of the optical member. On the other hand, the organic film, which has the carbon fluoride group, absorbs the short wavelength laser beam such as KrF and ArF excimer lasers, although the transmittance is high with respect to the relatively long wavelength light beam such as the i-ray (365 nm) in the same manner as the titanium oxide film. Therefore, the transmittance is lowered. Further, the organic film having the carbon fluoride group itself is decomposed by the radiation of the short wavelength laser as described above.
If the optical member in the projection optical system is exchanged, the following problems arise as well. That is, the cost arises, which is required for the exchange. Further, it takes a certain period of time to adjust the projection optical system after the optical member is exchanged.
In particular, when the pulse laser beam is used, it is also conceived that the method for removing the pollutant such as organic matters adhered to the surface of the optical member is based on the use of the photo-washing in which the pollutant is evaporated by radiating the pulse laser beam. However, a problem arises such that the pollutant, which is formed by a kind of photo-CVD action by the pulse laser beam, is difficult to be removed by the photo-washing. Further, some types of pollutants cannot be removed even by the organic solvent washing with alcohol and/or ketone.